Prebiotic fibers are essentially the “water” and “fertilizer” that nourish your bowel flora.These are fibers that you ingest but cannot digest, leaving them for microorganisms in the intestines to consume. Some call prebiotic fibers resistant starch since they are impervious to human digestion and digested by microorganisms. Getting prebiotic fibers is crucial to your health and the success of your diet.
Don’t confuse prebiotic fibers with the more commonly recognized cellulose fibers from bran cereals, bran muffins, and whole grains, not too different from wood fiber. Cellulose is not metabolized by you or by bowel flora, thereby providing nothing more than bulk in bowel movements with none of the physiological benefits of prebiotic fibers. If you came to believe that bran products were the answer to health problems, you were once again fooled by the overly simplistic thinking and/or marketing of the food industry, an argument not too different from “you need more sawdust in your diet.” Cellulose is not harmful, but it does not provide the benefits of prebiotic fibers. The substantial benefits of fiber come via bowel flora digestion of prebiotic fibers to fatty acids and vitamins, not by having a larger bowel movement from cellulose.
The average (unhealthy) American obtains between 3 and 8 grams of prebiotic fibers per day, about half from grains. Measurable health benefits begin at a prebiotic fiber intake of around 8 grams per day, while maximum benefits occur at an intake of 20 grams per day. You should therefore aim to obtain 20 grams each and every day to stack the odds in favor of cultivating a successful bowel flora garden. Now that you know this, what should you eat?
Here are the foods richest in prebiotic fibers:
Green bananas and plantains— And I mean green. Not green-yellow, or a little green at one end, but green. It will be tough to peel and virtually inedible, so slice it lengthwise, shell out the pulp, chop it coarsely, and then use it in one of the prebiotic shake recipes, I provide in Undoctored. You may have to stay alert for when the grocer puts out green bananas, and then either store them in the refrigerator, where they generally stay green for 4 to 5 days, or peel, chop, and store them in a container in the freezer and use as needed.
Potatoes— All potatoes when cooked are high in sugars and low in fiber. But when raw, white potatoes in particular are rich in prebiotic fiber—with 10 to 12 grams per one-half medium (31⁄2 inches in diameter) potato—and contain zero sugar. (Sweet potatoes and yams have far less prebiotic fibers. This means that you chance excessive carbohydrate exposure even when they are consumed raw. Eat only small quantities, whether raw or cooked.) Some people actually enjoy eating raw white potatoes like an apple, while others prefer to include them in prebiotic shakes.
Inulin and fructooligosaccharide (FOS) fibers— From chicory root, Jerusalem artichoke, and other sources, these fibers can be purchased from health food stores as a purified powder. (Inulin has a longer fiber chain, FOS shorter, but they exert similar or overlapping benefits.) Inulin and FOS are easily added to foods such as the “granola.” I provide a great recipe in chapter 10 of Undoctored.
Legumes— Kidney beans, black beans, white beans, chickpeas, other starchy beans, and lentils can be rich sources of the galactooligosaccharide, or GOS, form of prebiotic fibers. Hummus (pureed chickpeas) is another convenient source. However, legumes contain the carbohydrate amylopectin C, and while not as digestible as the amylopectin A of grains, it still has the potential to mess with blood sugars. We sidestep this issue while still obtaining a modest 3 to 4 grams of prebiotic fibers by limiting ourselves to small servings—1⁄4 to 1⁄2 cup cooked with no more than 15 grams net carbohydrates (total carbohydrates minus fiber). (Use your carb-counting resource to calculate for each form of legume. Don’t worry: This gets easy after a few tries.)
Modest quantities of prebiotic fibers (generally around 1 gram per serving) can also be obtained through peas, jicama, turnips and parsnips and other root vegetables, onions, and garlic, as well as apples, oranges, and carrots. Of course, always mind your net-carb counts on these foods.
In summary, try to include prebiotic fiber choices from the following list every day.
Green bananas and plantains: 10.9 grams per 1 medium (7-inch) banana (0 gram net carbs)
Raw white potato: 10 to 12 grams per 1⁄2 medium (0 gram net carbs) (Avoid any raw potatoes with green skin, as this is a fungus. If encountered, peel off the skin.)
Inulin and /or FOS powders: 5 grams per teaspoon (0 gram net carbs)
Hummus or chick peas: 8 grams per 1⁄2 cup (13.5 grams net carbs)
Lentils: 2.5 grams per 1⁄2 cup (11 grams net carbs)
Beans: 3.8 grams per 1⁄2 cup (white beans are the richest with twice this quantity) (12 grams net carbs)
Note: Values for prebiotic content vary depending on the source and the method used to measure.